1. Field of the Invention
The present invention relates to an apparatus for the production of a tubular thermoplastic film, and more particularly, relates to a water bath film cooling apparatus for the rapid quenching of an extruded tubular thermoplastic film so as to enhance the speed of cooling thereof, and increase the strength of the film material. Furthermore, the invention relates to a method of rapidly cooling a thermoplastic film material through the intermediary of a water bath, utilizing the inventive cooling apparatus.
The concept of utilizing a water bath for the cooling of a molten tubular film subsequent to the extrusion thereof is well known in the technology. In a more specific physical application, the tubular film is adapted to be extruded upwardly into a liquid bath, whereby the liquid, such as water, is arranged to externally encompass and contact the surface of the molten thermoplastic tubular film material. A concurrent flow of pressurized air which is supplied to the interior of the extruded molten film tube approximately balances the hydrostatic head of the liquid bath at generally the level of the extrusion die orifice. As the film tube is conducted upwardly, the hydrostatic head of the liquid, decreases, whereas the air pressure internally of the tube is maintained at a constant level so as to, resultingly, increase the pressure differential between the interior and the exterior of the film tube. Consequently, at some point along the upward path of travel of the tubular film, the pressure differential becomes sufficiently large so as to stretch the film and thereby orient the latter. In contrast with the foregoing, sheet film orientation is usually carried out by casting the base sheet into a water bath or onto a chill roll, thereafter stretching the film in the machine direction (MD) by superposing the film on successively more rapidly rotating rolls, and then stretching of the film in the transverse direction (TD) through the employment of a tenter frame.
Orientation of a tubular film web is, quite frequently implemented through the intermediary of a so-called double bubble. The base film is produced by extruding a tube through an annular extrusion die and thereafter freezing the molten tubular film by being contacted with water. Thereafter, the cooled tubular film is passed through a nip roll in order to isolate the internal pressure reigning in the tube, and then heated and inflated by a gaseous high-pressure medium into a second bubble. One method of obtaining the higher pressure in the second bubble is to employ a tubular probe extending through the mandrel of the extrusion die, and then through a notch which is formed in an intermediate nip roll. An alternative method resides in capturing a second bubble between two pairs of nip rolls, then moving the second pair closer to the first pair so as to reduce the length of the air column within the bubble, causing an increases in the internal pressure and laterally expanding the bubble which is constituted from the tubular film.
In essence, various thermoplastic film materials, particularly such as, but not limited to, polypropylene, polystyrene, polyester and high-density polyethylene can have their tensile strengths substantially enhanced through orientation. A small amount of orientation can be readily achieved through the bubble blowing process; however, any optimized degree of orientation can generally only be carried out and attained through a rather complex process, such as the above-mentioned double bubble extrusion, or with the application of a tenter frame.
The inventive method and apparatus is adapted to produce a high-clarity oriented tubular thermoplastic film at a high rate of production speed, with considerably less complexity than that currently employed in the technology, and may have ultimate commercial applications to plastic bag production or possibly consumer-oriented shrink film.
An orientation process for crystalline polymer films of the type considered herein may essentially follow at least some of four process steps:
1. Quenching the molten film material rapidly to approximately ambient temperature or even lower. PA1 2. Heating the film material to an elevated temperature which is still significantly below the melting point. PA1 3. Stretching the film material. PA1 4. Heat setting the material by heating the film to a temperature just below the melting point and maintaining the temperature for a few seconds.
The purpose of Step 1, in essence, the rapid quenching or cooling of the film, is to quickly conduct the film material through the temperature at which any encountered crystallization is most rapid (immediately below the melting point) in order to minimize the crystallinity. This particular rapid cooling step is necessary inasmuch as a high degree of crystallinity imparts a high strength to the film material, consequently rendering it difficult to orient the film without causing fracture thereof.
The intent of Step 2 is to avoid the presence of any line drawing of the film material by heating the latter to above room or ambient temperature, the line drawing being essentially a discontinuity or an area of highly stretched film which is surrounded by a region of unstretched film material. This may occur when one particle of the film reaches the peak of a stress curve prior to adjacent thicker particles in the film which possess a lower stress because of their greater thickness. Once the thinner film region has passed the peak, the forces drop off, as a result of which the thicker particles are never pulled over the peak until the original particle is stretched far enough. When the film is heated, the stress-strain curve during the drawing thereof changes in a manner so that all points of the film are stretched continuously.
Step 3 merely relates to the stretching of the film in any normal manner well known in the art; whereas the purpose of Step 4; in effect, the heat setting, the latter of which is not essential for all types of oriented thermoplastic films, is primarily to stabilize the film so as to present the material from heat shrinking.
The present invention is essentially directed to the provision of a unique method and apparatus for imparting an improved degree of control over the bubble pressure reigning within the tubular film by bubbling air through a variable depth column of water, thereby facilitating a rapid quenching through an external water bath with a highly accurate regulation over the cooling conditions to thereby enable an increase in the speed of cooling and in the strength of the film material by minimizing the crystallinity thereof during the quenching or cooling sequence which is imparted to the molten film tube.
2. Discussion of the Prior Art
Although numerous apparatus and processes are currently known and employed in the thermoplastic tubular film extrusion technology which pertain to the cooling or quenching of a molten film subsequent to extrusion thereof; and with various of these being directed to cooling or quenching aspects through the intermediary of a water bath and the like, none of these enable the rapid cooling with an enhanced degree of control over the pressure differential existent between the internal and external surfaces of the tubular film.
Sims, Jr., et al. U.S. Pat. No. 4,203,942 disclose a process and apparatus for producing a tubular thermoplastic film, in which the film is drawn downwardly from an annular extrusion die through a cooling liquid bath prior to being collapsed between cooperating nip rolls. There is no disclosure of the inventive control between the internal and external pressure differential acting on a tubular film as it is advanced upwardly through a cooling or quenching water bath analogous to that contemplated by the present invention.
Stelmack U.S. Pat. No. 4,115,047 discloses an apparatus for quenching blown tubular film by passing a tubular film material through a liquid bath. Again, there is no precise correlation of the internal and external pressure differential which would allow for the rapid quenching and resultant increased strength in the film material without encountering the adverse effects of producing a premature film crystallinity.
Okada, et al. U.S. Pat. No. 4,308,192 disclose a method and apparatus for forming a double-layered film, in which each separate film layer is passed through a liquid bath subsequent to extrusion. There is no utilization of pressure differentials within and externally of a tubular film material analogous to that contemplated by the invention in order to improve upon the production efficiency.
Moreover, various similar types of water or liquid baths for cooling molten thermoplastic film are described in Trull U.S. Pat. No. 2,488,571; Wiggins, et al. U.S. Pat. No. 3,280,233; Uemura, et al. U.S. Pat. No. 3,932,080; Johnson U.S. Pat. No. 3,821,339; and Reichel, et al. U.S. Pat. No. 2,337,927, among numerous other publications pertaining to this technology. None of the prior art patents are adapted to provide for the accurate control and interrelationship between the internal pressure of the extruded tubular film and an external water bath so as to optimize the rapid quenching conditions providing for an increase in the cooling speed of the film and concurrently strengthen the film without adversely affecting the crystallinity thereof.